Pigments and/or fillers based on particles in the nanometer range (so-called nanoparticles) such as, fore example, nano-calcium carbonate are known and are used in numerous applications including paper, paint and plastics applications. Organic and/or inorganic pigments and/or fillers in the micrometer range (so-called microparticles) such as hollow spheres or solid particles based on polystyrene, and inorganic mineral particles such as talc-, mica- or kaolin based pigments and/or fillers are also known and are used in the same or similar applications.
Mixtures of nanoparticles and microparticles of different chemical compositions are used because they have certain different properties which are advantageous to combine to impart the desired properties to the end product, e.g., paper. Mixtures of such substances are used, e.g., as pigments or fillers in papermaking but especially in paper finishing as in coating, e.g., to improve the quality of the paper with regard to the opacity, whiteness and gloss of the paper or the printability and printing properties. It is known that the properties of such microparticles and nanoparticles with regard to retention in papermaking and coating “holdout” in paper finishing, e.g., paper coating can be combined advantageously. Coating holdout is understood by those skilled in the art to refer to whether the coating remains on the paper surface or penetrates partially to completely into the paper surface or whether a portion, e.g., the binder and/or a pigment or a partial fraction of a pigment is segregated from the whole and penetrates into the paper surface. This is a problem with which those skilled in the art are familiar, especially in coating an absorbent substrate using coating colours with a low solids content.
When using mixtures of such microparticles and nanoparticles in such applications, an unwanted separation of components, so-called segregation, unfortunately occurs frequently and is associated with an uneven distribution of the coating with regard to the coating thickness on the surface underneath, the underlying pre-coating or the paper surface, which may thus lead to an uneven printing on the paper, for example. The term “segregation” refers to the process of separation of different elements in an observation field with a tendency toward spatial distribution of the elements according to certain properties.
Segregation of pigment and/or filler mixtures results in differences in pore volume in the coating, e.g., in finishing the paper by coating, because the free nanoparticles become segregated from the microparticles and therefore may either occupy the pores of the paper and/or the coating or “float” there, i.e., collect primarily in the upper area of the coating, for example, which is important in particular when the coating should absorb a certain volume of liquid such as water, oil and/or organic solvents from the printing ink in the subsequent printing.
A number of such mixtures, their production and use are known in the state of the art, describing mainly calcium carbonate as nanoparticles.
A widely used technique for producing such pigment or filler mixtures is described in DE 33 12 778 A1 and DE 43 12 463 C1, for example, and consists of mixing and joint grinding of a mineral filler such as natural calcium carbonate with a mineral filler such as talc.
However, under the conditions of papermaking or coating, such mixtures are usually subject to segregation because the bonds between the components of the mixture often do not withstand these conditions. It is known that shear rates of more than 106 sec−1 may occur in coating with the doctor blade at 1500 m/min.
Therefore, additional methods for producing such composites have been developed based on crosslinking between the pigment and/or filler particles, where numerous internal cavities are formed that should improve the physical properties and especially the optical properties of the pigments and/or fillers.
Thus, a method for forming chemically aggregated porous pigment composites is described in WO 92/08755, where an aqueous slurry of mineral particles such as calcium carbonate is prepared and a polymer or copolymer containing carboxylic acid groups is added to the slurry to cause flocculation. Calcium ions are added in excess to the slurry to induce precipitation of the calcium salt of the polymer on the mineral flocks and therefore produce aggregates of the mineral particles that are bonded by the calcium salt and have a porous flaky structure. The excess calcium ions are reacted with carbon dioxide and precipitated as calcium carbonate on the polymeric calcium salt. However, since the calcium ions are added in the form of alkaline chemical compounds such as calcium hydroxide, they form alkaline intermediates that can have negative effects, e.g., when using certain dispersants. In addition, further precipitation of calcium carbonate alters the structure of the original nanoparticle/microparticle structure and necessarily leads to the introduction of another pigment, namely the precipitated calcium carbonate formed by neutralization. Flocculated aggregates can be problematical in general in paper applications because they cause diffuse light scattering on the surface which leads to loss of the paper gloss. In addition, the pore volume of the composite to be achieved originally is influenced and altered first by the flocculation and secondly by the precipitated calcium carbonate thus formed.
U.S. Pat. No. 5,449,402 describes functionally modified pigment particles that are produced by mixing of flocculated pigments such as calcium carbonate with a regulator substance having an opposite charge from the charge of the flocculated pigment. The flocculated pigment is preferably an aqueous suspension of filter cake particles. Preferred regulator substances include water-insoluble or dispersible latex binders, water-soluble or alkali-soluble organic and/or inorganic polymer binders and non-film-forming organic particles that are electrostatically bound to the pigment particles when mixed with them.
U.S. Pat. No. 5,454,864, U.S. Pat. No. 5,344,487 and EP 0 573 150 also describe pigment composites whose production is based on electrostatic attraction forces between the carrier particles and the coating particles. However, the use of such composites may be problematical in the respective applications because of interactions with other charged components.
Another method for improving whiteness according to WO 97/32934 consists of coating the pigment particles with other pigment particles such as finely divided particles of precipitated calcium carbonate which are initially present in the form of agglomerates, but without using a binder, which can lead to the problems mentioned above such as flocculation. The stability of these composites is based essentially on the forces of attraction such as van der Waals forces that can develop only when certain very specific conditions are met. For example a defined pH must be maintained exactly to obtain the best possible zeta potential, which is different for each combination of substances. As soon as the conditions deviate from the optimal, the forces of repulsion become predominant and the components undergo segregation.
WO 99/52984 pertains to composite compositions of costructured or coadsorbed fillers which contain at least two different types of mineral or organic fillers or pigments, e.g., from calcium carbonate, talc or polystyrene and use thereof. The different types of pigments or fillers have hydrophilic and/or organophilic regions which allow binding to take place by way of special binders. The binders, which must have an affinity for the hydrophilic components as well as the organophilic components to manifest their binding function, are selected from special polymers and/or copolymers. The particle diameter of the pigments and/or fillers used does not play a role here inasmuch as no diameter is mentioned explicitly and/or all the particle diameters mentioned in the examples are less than 1 μm in the best case.
WO 03/078734 discloses a composition for surface treatment, in particular for coating paper, containing a nanoparticle fraction, e.g., of precipitated calcium carbonate, and a carrier fraction comprising platelet-like pigment particles, including talc or plastic pigment particles and at least one binder. However, the nanoparticles do not coat the carrier. By targeted arrangement of the platelet-like microparticles on the paper surface, pores are closed and nanoparticles can no longer penetrate. It is describes how the platelet-like microparticles migrate to the paper surface due to segregation and thereby close pores between the fibres and thus prevent the nanoparticles from being able to penetrate into the surface. Thus targeted segregation of nanoparticles and microparticles is a goal. Microparticles segregate from the nanoparticles and are situated at the bottom of the coating while nanoparticles are at the top of the coating. The binder, preferably a polymer latex binder, causes the bond to form between individual particles and the two particle fractions at the top and bottom of the coating when the coating dries on the paper. The desired segregation has already taken place at this point in time.
US 2005/0287313 relates to the subject of fusible print media based on a substrate and an ink-absorbing layer on the substrate. The ink-absorbing layer comprises a plurality of hollow spheres, e.g., polystyrene hollow spheres which have essentially the same diameter which may be 0.3 to 10 μm. The layer also includes binders such as polyvinyl alcohol or polyvinylpyrrolidone and the like to bond the hollow spheres together. The hollow spheres may also be partially replaced by microporous and/or mesoporous inorganic particles such as calcium carbonate or talc as well as polymer particles that are not hollow and may have a diameter of 0.2 to 5 μm. US 2005/0287313 thus describes a mixture of microparticles that are present concurrently and held together by fixation in a binder tailored to the requirements of the melting process. It is a type of pickling bath which may consist of certain cationic polymers and copolymers containing amino groups and is fed to ensure a better chemical interaction between a dye-based ink and the ink absorbing layer. It does not play any role with regard to the binding of the different components within the layer. The problem of segregation is not mentioned.
WO 2006/016036 relates to, among other things, a method for grinding mineral materials in water in the presence of binders and the resulting suspensions as well as the use thereof in coating formulations. A large number of materials such as talc that can be ground in the presence of binders are mentioned in the description and claims. However, the examples use only calcium carbonates. In none of the examples grinding of, for example, two chemically different materials in the presence of a binder is disclosed. Furthermore, there is no mention of the fact that nanoparticles are formed or nanomicrocomposites are produced by this grinding method. The binder is not used to produce a composite but instead as a grinding aid for finer grinding, but the average diameter of the particles in the pigment suspensions may be up to 30 μm. The binders used for grinding may be based on styrene-acrylate or styrene-butadiene, i.e., these are binders with which those skilled in the art are well familiar such as those used in coating papers or as binders in wall paint. Thus, the method described in WO 2006/016036 obligatorily includes a grinding step which yields particles essentially in the micro range and it does not describe a binder that allows the formation of an essentially segregation-resistant composite.
DE 10 2006 026 965 describes composites comprising inorganic and/or organic pigments and/or fillers in the form of microparticles, the surface of which is coated at least partially with the help of binders with calcium carbonate particles in the nanometer range, a method for the production of such composites, aqueous slurries thereof and their use in the paper production or in the field of paint and plastic production and the use of these binders for coating the microparticles with nano-calcium carbonate. The use of dolomite particles is not mentioned.
WO 2006/033952 describes a material of paper and board comprising a substrate of paper or board, a basic layer on top of at least one substrate surface and a top layer on top of a surface of the basic layer, wherein said top layer comprises one or more pigments being dispersed in one or more binders, and said basic layer comprises thermoplastic particles having a low density such as the plastic particles Ropaque® HP-1055 and AF-1353 of Rohm and Haas and HS 2000NA and HS 3000NA of Dow Chemical Company, which are dispersed in one or more binders. The basic layer is compressible reducing the back trap mottling in offset printed pictures. Pigment particles such as calcium carbonate and plastic particles thus are present in separate layers. All of said compositions of the state of the art particularly describe calcium carbonate as one component. Calcium carbonate, however, is not acid-resistant and decomposes under the influence of acids forming carbon dioxide and calcium salts of the corresponding acid used. Many, especially paper applications and manufacturing processes such as fibre bleaching are performed in an at least slightly acidic environment such that the sensitivity to acids of calcium carbonate in such procedural steps may represent a significant problem. For example, water hardness thus can be extremely increased, which in turn can lead to deposits later in the paper manufacturing process. It is also possible that acidic food such as fruit, e.g. lemons are exposed to packaging paper, which may lead to an undesired deposit of calcium salts on the food. In addition filtering of liquid acidic food and beverages such as vinegar is not possible with the materials mentioned above being acid sensitive.